Stacked in police warehouses across the Valley, there are thousands of rape cases with DNA samples that have never been tested.

Experts tell the ABC15 Investigators it's critical evidence that is being ignored by law enforcement, leaving suspected rapists on the street and victims without answers.

"I think you're looking at a jurisdiction that's not responding effectively to sexual assault," said Sarah Tofte, a national advocate with the Joyful Heart Foundation, who studies how police handle rape case evidence.

The ABC15 Investigators spent four months researching this story, sending out dozens of public records requests and reviewing hundreds of pages of documents, police reports and court records.

Beginning on Monday during ABC15 News at 10, we will specifically focus on whether or not police have tested evidence in what's called a "sexual assault evidence kit."

When victims report rapes or sexual assaults, they are given an exam and swabbed all over their body to collect a suspect's DNA left by semen, saliva or other bodily fluids.

The DNA evidence collected is stored in those kits,which are kept in freezers at Valley police storage facilities.

ABC15 Investigator Dave Biscobing has discovered there are thousands of sexual assault kits that have never been tested.

"An untested kit, for us, is about lost justice for rape victims," Tofte said.

DNA that is gathered and stored in sexual assault evidence kits can be entered into state and federal databases and matched directly to DNA profiles of convicted offenders and arrestees, including suspects in other rapes.

So if a kit isn't tested, police lose the chance to connect that DNA to other attacks, someone who's already been arrested or someone who will be arrested in the future.

During our series of reports, the Investigators confront top law enforcement officials with the numbers, share emotional stories from victims and their families, and show how a serial rapist could have been stopped earlier if DNA evidence had been tested.

The first report of our exclusive ABC15 investigation starts Monday on ABC15 News at 10 p.m.